Intravenous fluid or drug additive systems have been used in patient care for many years. The systems typically operate by gravity induced flow from a container positioned above the patient.
Traditionally, the flow rate of the fluid from the container was measured with a drip chamber at the container and controlled by a clamp varying the restriction in a delivery line extending between the drip chamber and the patient. A nurse or aide visually counts the drop rate in the drip chamber and manually sets the clamping or pinching device to achieve a desired flow rate. The accuracy of this system assumes the consistency in volume of each drop of fluid into the drip chamber. However, the drop size is dependent on the orifice diameter in the drip chamber which varies within a manufacturing tolerance for a particular container. Even the nominal orifice diameter is not uniform throughout the industry. Manufactures sell drip chambers having 10, 15, 20 and 60 drop per milliliter chambers, for example. In addition the volume of the drops may vary with temperature, viscosity and rate.
The pinching device may also induce error in the flow rate. For example, the tubing adjacent the pinching device may relax, altering the restriction to flow and permitting variation in the flow rate from the desired value. The gravitational head pressure acting to infuse the fluid into the patient may also vary. For example, the patient may turn over, sit up or roll on the delivery line to further restrict flow.
One attempt to overcome several deficiencies in delivery systems including a drip chamber is disclosed in U.S. Pat. No. 4,204,538, issued May 27, 1980 to Raymond E. Cannon. This patent discloses a cassette which provides for controlled introduction of fluid from a container to the patient. A chamber is provided which is divided into two compartments by a flexible separating member. The separating member is attached at its periphery to the walls of the chamber and is movable between the end walls defining the boundaries of the compartments. Conduits lead to each of the compartments from a container with a valve positioned in each conduit. Branch lines extend from each of the compartments to the patient with a valve being positioned in each line. The valve operation is sequenced so that fluid flow comes into one chamber from the container while fluid leaves the other chamber for delivery to the patient. The separating member flexes to enlarge the volume of the filling compartment and decrease the volume of the compartment flowing to the patient. However, the rate of flow of the fluid is controlled by setting a pinch clamp.
The construction of the cassette includes four individual segments in facing relationship. Each of the segments has a complex shape. This results in high manufacturing costs. Each of the valves positioned in the cassette are positioned for activation by parallel reciprocating rods. The cassette must remain stationary while each rod activates its associated valve.
A need has arisen for a metering unit which overcomes the problems associated with the drip chamber and variation in drip volume. Finally, a metering unit is needed which accurately performs the function of metering fluid to the patient in a cost effective manner.